I believe that i'm quite familiar with the basic CP antenna design such as CP patch antenna. Two orthgonal mode with equal amplitude and 90 deg phase difference.

However, there's one question in my mind for long time - Can a mechanically rotating (say rotating at very high speed) dipole generate CP radiation? If yes, how to determine the sense of rotation (RH or LH)? If no, what is the polarization then?

My though/answer is "it can not generate CP" but i don't have simple explanation, and want to listen somebody is thinking.

Lets say we have a dipole antenna that radiates at 100 MHz, and is linearly polarized vertically.

A circularly polarized field would rotate at 100 MHz as well. Suppose you tried to spin a dipole at 100 MHz. You would not end up with circular polarization.

The reason: the current on the dipole antenna is also oscillating at 100 MHz. If at the start of the cycle the current is at a peak value and the dipole is vertically oriented, then a quarter of a cycle later the current will have zero magnitude and be horizontally oriented. However, to achieve circular polarization you would need the antenna to be radiating horizontally-polarized fields at this point, but it would have no current, so it wouldn't be radiating anything.

If you could somehow get a d.c. current to flow on a dipole antenna and then spun it at 100 MHz, maybe that would work. But if its a typical dipole antenna that is terminated in an open circuit, no d.c. current will flow.

Assuming you could get a d.c. current to flow on something, and spin it at 100 MHz, you could possibly get CP radiation out of it, I am not sure. Let me know if you try it.

Well the spinning dipole would receive some of the radiation, but would probably lose half the available power due to polarization mismatch (-3 dB). I have no idea what the exact polarization mismatch loss would be.